Not Applicable
1. Field of the Invention
The present invention relates generally to centrifugal pumps and mixers. More particularly, the present invention relates to centrifugal pump and mixer shear force rotors.
2. Description of the Related Art
Centrifugal pumps have been known for a number of years. A centrifugal pump is a device that converts driver energy to kinetic energy in a liquid by accelerating it to the outer rim of a revolving device known as an impeller. The impeller typically includes two xe2x80x9cshroudsxe2x80x9d that together form a fluid flow channel. Impellers also typically include xe2x80x9cvanesxe2x80x9d extending between the shrouds. Vanes are relatively thin, rigid, flat, and sometimes-curved surfaces radially mounted between the shrouds. The vanes are similar to a blade in a turbine and are used to turn the fluid. The amount of energy given to the liquid corresponds to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller, the higher the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.
As the impeller revolves, it imparts an external force on the fluid. The external force circulates the fluid around a given point to create xe2x80x9cvortex circulationxe2x80x9d. As the external force circulates the fluid, it accelerates the fluid in the tangential direction as the fluid moves outward. Circulating the fluid thus maintains the angular velocity of the fluid. The external force accelerates the fluid by transferring momentum from the impeller to the fluid.
The vortex circulation also creates a radial pressure gradient in the fluid. The gradient is such that the pressure increases with increasing radial distance from the center of rotation. The rate of the pressure increase depends upon the fluid rotation speed and the density of the fluid being pumped.
There are a number of shortcomings associated with standard centrifugal pumps using a traditional impeller in viscous liquids. These deficiencies seriously limit the application range for centrifugal pumps. Many of the problems occur in the impeller eye where the fluid is first introduced into the impeller. The detrimental impact of these difficulties is that a conventional impeller can run in cavitation and extremely low efficiencies when pumping viscous fluids. A conventional impeller can also have high abrasive wear when pumping abrasive fluids. Although some of these downfalls can be overcome by modifications to the pump and the pumping system, such modifications are costly.
When impeller vanes of a centrifugal pump travel through a fluid, they produce a pressure distribution that has a positive pressure on the forward face of the vane and a negative pressure on the backside of the vane. The intensity of the negative pressure zone depends on the radial flow velocity of the fluid behind the vanes and the rotational velocity of the impeller. This type of pressure distribution is inherent in a pump utilizing a vaned impeller.
Cavitation can occur in the negative pressure zone in the area having the lowest static pressure. In a standard vaned impeller, the lowest pressure is at the fluid inlet, and more specifically on the rear side of the vane at the fluid inlet. If the static pressure on the fluid in the pump drops below the vapor pressure for the fluid, vapor pockets will be formed. Cavitation occurs when the vapor pockets move from the low-pressure zone to the high-pressure area and implode. Cavitation severely restricts the performance of the pump.
In order to avoid cavitation, suction pressure must be increased so that even the low-pressure areas at the impeller inlet have sufficient pressure. Increasing suction pressure causes the static pressure to be higher than the vapor pressure of the fluid. It is very expensive, however, to provide additional inlet pressure to a pump to suppress cavitation. Also, the location in which the pump is being used may not allow for the alterations required to increase the inlet pressure.
Simply stated, with traditional impeller designs, viscous liquids like heavy oil, highly concentrated slurries, and sludges are not able to accelerate quickly enough to fill the voids created behind the vanes of a rotating impeller. This causes the pump to cavitate and in some instances, stop pumping totally.
Traditional centrifugal pumps also experience shortcomings with respect to abrasion. When pumping abrasive slurries, the rate of wear is a function of the type and the concentration of the solids in the slurry and the velocity between the surface of the impeller and adjacent fluid layer. There is a layer of relatively dormant fluid, called the boundary layer, next to the surfaces of the impeller. The Reynolds number of the fluid determines the thickness of the boundary layer. The boundary layer provides a protective layer of fluid that helps prevent the abrasive slurry particles from coming in contact with the surface of the impeller. However, the shielding by the boundary layer is somewhat reduced when the thickness of the boundary layer is decreased. The effects of the abrasive slurries are greatest at the impeller xe2x80x9ceyexe2x80x9d where the fluid undergoes abrupt acceleration and changes of direction. Thus, when pumping abrasive fluids, the inlet region of the impeller will receive the most harm and be the first area of the impeller to fail.
Traditional centrifugal pumps also experience shortcomings due to having parallel disks disposed co-axially in the rotor chamber as the shrouds. Parallel disks limit the pump design to radial flow designs only and can cause tip cavitation, which is destructive to the disc impeller. Also, the spaces between the discs and the pump case are extremely high shear areas that cause a dragging force. With viscous liquids, the extremely high shear areas generate a breaking action, making these pumps very inefficient. The inefficiency results in higher horsepower requirements and also higher costs.
Traditional centrifugal pumps also experience shortcomings due to raised vanes on the impeller with square or rectangular cross sections that extend radially from the periphery (outer diameter) of the impeller towards the center. This causes a problem with abrasive slurries on the trailing side of the raised vane. The square section creates an eddy behind the vane that accelerates the abrasive solids in a slurry. Accelerating the solids induces wear, which ultimately cuts holes through the disc directly behind the vane.
Raised vanes that extend into the xe2x80x9ceyexe2x80x9d, or center area, of the impeller are also problematic because the liquid transfers from laminar flow to turbulent flow as it enters into the xe2x80x9ceyexe2x80x9d. This causes two problems. First, in abrasive slurry service, the additional turbulence of the liquid entering the eye creates wear. This wear causes premature failure of the disc impeller. Second, when pumping fragile products, such as crystals, which are damaged due to shear and/or turbulence, the losses of product are very high and costly.
Some traditional centrifugal pumps also experience shortcomings because they do not incorporate close tolerance wear rings. Under high suction conditions, this allows recirculation from the exit port of the impeller, down the outside of the impeller shrouds, and back to the inlet area. This design oversight makes it impossible to perform a valid NPSHR test that is required by many users.
The embodiments relate to a single stage centrifugal pump rotor. The rotor is a duplex, shear force rotor designed specifically for pumping heavy oil and any other viscous fluids or abrasive slurries. One of the embodiments of the rotor includes two non-parallel shrouds that form a fluid flow channel between their inner, opposing faces. A plurality of short, raised ribs are included between the two rotor shrouds. The raised ribs radially extend approximately 50% of the distance from the outer perimeter of the rotor towards the eye of the rotor. The duplex, shear force rotor design includes an unobstructed inlet, or charging, section that imparts ample pressure and velocity to force-feed the discharge section of the rotor. With the duplex, shear force rotor, the slurry liquid enters the suction eye in a smooth laminar flow, unobstructed by the inlet ribs.
Another embodiment of the rotor is designed for use in a multi-stage centrifugal pump. The design of the rotor allows the drive shaft to extend completely through the rotor for powering engagement with additional rotors.
In another embodiment of the rotor, the shrouds of the rotors are both angled from perpendicular with the axis of rotation. The angle of the shrouds can be designed for handling mixed and semi-mixed fluid flow. The rotor may also be mounted on a cantilever shaft in a mixer for mixing, agitation, blending, and keeping solids in suspension.
Another embodiment of the rotor does not include raised ribs between the rotor shrouds.
Another embodiment of the rotor includes raised ribs with tapered trailing edges.
Thus, the embodiments comprise a combination of features and advantages that enable them to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings.